1. Field of the Invention
The present invention relates to the field of power electronics. It relates to an MOS-controlled thyristor MCT, comprising
(a) a semiconductor substrate having two opposite principal surfaces, one of which is assigned to an anode, and the other is assigned to a cathode and forms a cathode surface;
(b) a layer sequence inside the semiconductor substrate between the anode and the cathode, which layer sequence comprises an emitter layer of a first conductivity type, a first base layer of a second conductivity type opposite to the first, and a second base layer of the first conductivity type;
(c) a multiplicity of adjacently disposed and parallel-connected MCT cells inside the semiconductor substrate between the anode and the cathode;
(d) an emitter region of the second conductivity type embedded in the second base layer inside every MCT cell on the cathode side, connection to which emitter region is made from the cathode surface by means of a cathode contact; and
(e) an MOS structure, which forms a switchable short circuit between the second base layer and the cathode contact, inside every MCT cell on the cathode side.
Such an MCT is disclosed, for example, by the paper by V. A. K. Temple in IEEE Trans. Electron Devices, vol. ED-33, pages 1609-1618 (1986).
2. Discussion of Background
In the application in power-electronic circuits, in particular in speed-controlled motor drives, major system simplifications would be feasible if the current control in the power semiconductors used, as it is currently known from the GTO, could be replaced by a voltage control. For lower powers, this change from current to voltage control has already been achieved by replacing the conventional bipolar transistors with the recently developed IGBTs (Insulated Gate Bipolar Transistors).
For higher powers, which essentially remain the province of the thyristors, efforts have been in progress for a fairly long time to replace the GTO in a similar manner by developing the voltage-controlled MCT (MOS-Controlled Thyristor). Hitherto, however, these efforts have not been very successful because large-area MCTs continue to suffer from detrimental inhomogeneous current distributions, in particular during the turn-off phase.
Even for components having cathode areas of only one mm.sup.2, this phenomenon reduces the turn-off current densities to very low values of about 50 A/cm.sup.2. In contrast to this, for arrangements of only a few cells (a large-area MCT always comprises a multiplicity of individual small MCT cells in which the cathode area is then only about 0.01 mm.sup.2) excellent turn-off current densities of several 1000 A/cm.sup.2 are observed.